Agree with Rodman9999. You can use tuner, CD player, iPod, satellite radio or similar source into one of the high level inputs of your preamp. The exception would be breaking in the moving coil section of a phono preamp.
For that job, the Granite Audio CD (CD#101) with low level reverse RIAA signal is perfect for the job. The phono section thinks your running a moving coil cartridge, but without wearing out your cartridge or tending a turntable. The CD is easy to use and easy to pile up the hours by setting the appropriate track to repeat.
If you have only a super high end digital, perhaps one that's tube based and can't stand to see the hours pile up, buy a cheap one at Walmart or Best Buy for $90.00 and keep it around just for breaking in cable and equipment.
I have an old Rotel that works great for the job, Paid $40.00 for it. I bought the Granite Audio break in CD at one of the CES shows I was covering for Audiogon. I don't sell them or know who carries Granite Audio products but the CD is still listed on their web site.
Good luck with breaking in the preamp, it's a smart thing to do, you will be rewarded with more great listening time rather than suffering through the in between phase.